1. Field of the Invention
The present invention relates to a method of and device for detecting an image in an auto-focusing apparatus in which focusing condition is judged by using an image sensor, such as a CCD line sensor.
Various auto-focusing methods have been proposed, in which a CCD line sensor or like image sensor is used to detect an image. In order to ensure proper auto-focusing operation, an image zone relevant for auto-focusing operation should be projected so that the image sensor receives the light projecting through the such image zone. For instance, if an image zone having a monotoneous brightness or a substantially constant density is projected to be sensed by the image sensor, precise auto-focusing operation cannot be performed leading to erroneous operation. Accordingly, prior to performing auto-focusing operation, it becomes necessary to judge whether or not the image zone sensed by the image sensor is a zone relevant for auto-focusing operation.
2. Prior Art Statement
One known method of judging whether or not the image zone sensed by the image sensor is a zone relevant for auto-focusing operation or not is the method wherein the output signal from the image sensor is binarized (bilevel coded) by a predetermined threshold level to know the reversing number indicating the number of times for the image signal to be changed from a value under/over the threshold level to a value over/under the threshold level (in other words, the number of times for the sensed image zone to be changed from black to white or white to black), and it is judged that an image is present in the sensed image zone when the reversing number is more than a programmed number. However, in this known method, it becomes necessary to determine the density of the background, since the threshold level for binarization must be shifted depending on the density of the background and also depending on whether the original is a negative or positive picture.
Another known method involves the steps of pre-scanning the entire area of the original to make a histogram, and determining the density of the background by referring to the histogram. However, this known method is detrimental in that the time costed for the pre-scanning operation is redundantly long since the entire area of the original image must be pre-scanned for making the histogram. Although it is expected to save the time for pre-scanning by decreasing the area to be pre-scanned, the accuracy in determination of background density is lowered.
The accuracy is seriously lowered particularly when the background density is determined by pre-scanning an image zone having a relatively high density, e.g. an image zone containing a row of letters, and the slice level (threshold level) for binarization is set by referring to the thus determined background density.